First, it puts everyone involved at the same level-there are no black belts of dojo cleaning. It also teaches people responsibility for and commaraderie with their community. It shows respect for the teaching and the dojo and its environment as well as fellow students (no one wants to train in other people's sweat and dirt). It also demonstrates respect for the health of others (that dirt thing again).

Unfortunately, in some dojos where multiple groups train the following group often ends up cleaning the dirt from the previous group. Disrespectful and unconscionable IMHO.

Well, responsability, respect for the teaching, the environment and others health or safety is one thing. Humbling people is another thing.